1. Field of the Invention
The present invention relates to voltage-controlled oscillators used in various kinds of high-frequency circuits and communication apparatuses incorporating the same.
2. Description of the Related Art
A conventional voltage-controlled oscillator oscillating at two different frequencies is disclosed in Japanese Unexamined Patent Application Publication No. 11-168324. FIG. 4 shows the structure of the voltage-controlled oscillator described in this publication.
FIG. 4 illustrates the voltage-controlled oscillator including a resonance circuit 2, an oscillation circuit 3, a buffer circuit 4, and a matching circuit 5. The resonance circuit 2 operates to select one of two different frequencies, and resonates at the selected frequency. For example, the resonance circuit includes two resonance circuits resonating at different resonant frequencies, respectively, each of which selects one of the resonant frequencies. Otherwise, the resonance circuit includes inductors and capacitors. The resonant frequency of each resonance circuit is switched by switching the inductance or the capacitance. In FIG. 4, the reference character Vf denotes a switching signal used for allowing the resonance circuit to resonate at one of the two resonant frequencies.
The oscillation circuit 3 has a transistor TR1, a capacitor C7 connected between the base of the transistor TR1 and the emitter thereof, a capacitor C6 connected between the emitter thereof and a ground, and a capacitor C5 connected between the collector of the transistor TR1 and a ground.
The buffer circuit 4 has a transistor TR3. A signal output from the emitter of the transistor TR1 is input to the base of the transistor TR3 via a capacitor C8, and an output signal Vout is transmitted from the collector of the transistor TR3 via a capacitor C2.
The matching circuit 5 includes inductors L1, L2, and a capacitor C3. One end of the inductor L1 is connected to the collector of the transistor TR3, and one end of the inductor L2 is connected to a power source Vsc. A capacitor C12 and a diode D2 are connected between a junction of the inductors L1 and L2 and a ground. A switching signal Vsw is supplied to the diode D2 via a resistor R10.
In the voltage-controlled oscillator having the above-described structure, an oscillation frequency to be used can be switched, for example, from the 800-MHz band to the 1.6-GHz band. In other words, the oscillation circuit 3 oscillates at a resonant frequency determined by the capacitors C5, C6, and C7, and the resonance circuit 2. Then, the transistor TR3 buffer-amplifies the oscillation-frequency signal to output as Vout. Then, according to the oscillation frequency, the diode D2 is switched ON/OFF by the switching signal Vsw so that the impedance of the matching impedance 5 is changed.
Such a voltage-controlled oscillator switching the oscillation frequency between the two oscillation frequencies is used in a mobile phone using two kinds of communication systems having different frequency bands. In order to miniaturize a communication apparatus by taking advantage of a single voltage-controlled oscillator adaptable to two frequencies, it is necessary to reduce the size of the entire structure including circuits disposed in the voltage-controlled oscillator.
However, the conventional voltage-controlled oscillator requires a switching circuit for switching the oscillation frequencies and also requires a switching circuit for changing the impedance of the matching circuit. Therefore, it is necessary for a substrate to have areas where components constituting such switching circuits are arranged. As a result, it is difficult to achieve both miniaturization and cost reduction of voltage-controlled oscillators.
In order to overcome the problems described above, preferred embodiments of the present invention provide a voltage-controlled oscillator that greatly reduces the number of parts used, the overall size, and the manufacturing cost. Also, preferred embodiments of the present invention provide a communication apparatus incorporating the novel voltage-controlled oscillator.
According to a first preferred embodiment of the present invention, a voltage-controlled oscillator includes a first resonance circuit and a second resonance circuit resonating at different frequencies, a first oscillation circuit amplifying an output signal from the first resonance circuit so as to oscillate, a second oscillation circuit amplifying an output signal from the second resonance circuit so as to oscillate, a buffer circuit amplifying the output signals from the first and second oscillation circuits, a first switching circuit for controlling oscillation which is connected to the first oscillation circuit, a second switching circuit for controlling oscillation which is connected to the second oscillation circuit, and a matching circuit having an impedance that is capable of being changed by switching between the first switching circuit and the second switching circuit to match with an oscillation frequency of one of the first and second oscillation circuits.
In the above-described unique structure, the impedance of the matching circuit is changed by switching between the oscillation-controlling first and second switching circuits. With this arrangement, it is not necessary to provide a separate switching circuit for switching an oscillation frequency and another switching circuit for changing the impedance of the matching circuit. As a result, the number of required components is reduced, and the required component areas on a substrate are thereby reduced. Thus, the entire size of the oscillator and the manufacturing cost thereof are greatly reduced.
According to a second preferred embodiment of the present invention, a voltage-controlled oscillator includes a resonance circuit resonating at one of two different frequencies, an oscillation circuit amplifying an output signal from the resonance circuit so as to oscillate, a buffer circuit amplifying an output signal from the oscillation circuit, a switching circuit connected to the resonance circuit to select one of the resonance frequencies, and a matching circuit having an impedance that is changed by switchover in the switching circuit to match with an oscillation frequency of the oscillation circuit.
As described above, since the impedance of the matching circuit is changed by the switchover in the switching circuit connected to the resonance circuit to select one of the resonance frequencies, it is unnecessary to provide a separate switching circuit for performing the switchover according to the oscillation frequencies and another switching circuit for changing the impedance of the matching circuit. Thus, the number of required components is reduced and component areas on a substrate are thereby minimized, such that miniaturization of the overall size of the oscillator and reduction in manufacturing cost are achieved.
According to a third preferred embodiment of the present invention, a communication apparatus includes at least one of the voltage-controlled oscillator of the other preferred embodiments of the present invention. For example, the communication apparatus modulates a transmission frequency according to transmitted signals (data) and switches between the frequency bands according to communication systems.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.